The Fish Keeper’s Guide to PH, GH, And KH

The Fish Keeper’s Guide to pH, GH, and KH pH, GH, and KH are terms commonly used in water chemistry, but there is a lot of confusion about them in the freshwater aquarium hobby. What …


The Fish Keeper’s Guide to pH, GH, and KH

pH, GH, and KH are terms commonly used in water chemistry, but there is a lot of confusion about them in the freshwater aquarium hobby. What are the differences between them and how can they affect our fish? This practical guide for beginners explains what these parameters mean, when you should test for them, and how to raise or lower their levels if needed.


pH (or Power of Hydrogen)

The pH of water measures the amount hydrogen ions present in it and can be used to determine how basic or acidic your water is. Pure water has a pH of 7.0 on a scale from 0-14. Acidic liquids such as orange juice or vinegar have a pH less than 7.7, while alkaline liquids like green coffee and soap have a higher pH.

What pH level is ideal for aquariums?

Most freshwater fish can tolerate pH levels from 6.5 to 8.0. Caridina crystal shrimp and South American fish prefer a lower pH level, while African cichlids prefer a higher pH. While it’s not necessary to maintain a certain pH level if you keep fish for pleasure, it’s important to know if you want to breed fish or raise fry.

How to Measure pH

Aquarium Co-Op Multi-Test Strips include a test for measuring pH, and we recommend using it as part of your tank maintenance routine. If you have any questions about your fish’s health or want to keep them at a particular pH level, you can test their pH. A pH crash can cause your fish to exhibit signs of stress like erratic behavior, lethargy and rapid breathing.

The bottom line is that the pH of a fish tank changes naturally throughout the day. Most fish will adapt to a stable pH without sudden spikes.

AquariumCo-Op multi-test strips let you quickly and easily measure pH and KH in just one minute.

KH (or Carbonate Hardness)

KH measures the water’s level of carbonates or bicarbonates. This has an effect on the water’s buffering capacity. This means that KH helps neutralize acids and prevents your pH from changing too rapidly, which is useful because sudden pH crashes can cause health issues in your fish. A low KH water level means that it has less buffering power and your pH swings more easily. High KH indicates that your water has a greater buffering capacity, and is more difficult to alter.

KH is like a trashcan. The trash can gets larger the higher KH. The pH crash happens when the trash can gets too full. Low KH tap water users often use crushed coral as a way to slowly raise it (or increase the trash can) and avoid pH crashes.

What is the Ideal KH Level for Aquariums?

KH is measured in dKH (degrees of KH) or ppm (parts per million), where 1 dKH equals 17.9 ppm. Typically, freshwater aquariums should be between 4-8 dKH (or 70-140 ppm). You can lower the pH of animals such as crystal shrimp or discus by lowering the KH to 0-3dKH (or 50 ppm). African cichlids, on the other hand, appreciate KH higher than 10 dKH (or 180 ppm), which usually goes hand in hand with higher pH levels.

How to Measure KH

The multi-test strips are great for measuring KH. They can be used as part of our routine water changing. You can find our guide on how often you should change your water. If you are trying to increase your KH level to avoid pH swings and b) if it is important to lower your KH, you might also need to measure KH.

Summary: Keep in mind that KH should not be below 2 dKH. This is because pH swings can easily occur and could potentially cause death for your animals. (The exception is if you’re raising certain animals that like low pH.) You can raise KH if you have low levels.

GH (or General Hardness)

GH measures the amount of calcium and magnesium ions in the water – in other words, how hard or soft your water is. It’s one of the easiest ways to measure if your aquarium water has enough salts and minerals that are essential for healthy biological functions, such as fish muscle and bone development, shrimp molting, snail shell development, and plant growth.

What is the ideal GH level for aquariums?

As with KH, GH is measured in dGH (degrees of GH) and ppm. Ideally, freshwater aquariums have a GH between 4-8 dGH (or 70-140 ppm). While all animals require minerals, certain species of fish, such as livebearers, goldfish and African cichlids, need them more. If you want to breed discus, or any other soft water fishes, the GH should be reduced to 3 dGH or 50 ppm.

How to Measure Gh

We recommend using the multi-test strips if you’re trying to reach a certain GH level or if your animals and plants are showing health issues. Symptoms of low GH include:

– Fish that are unable to eat, have slow growth rates, or show signs of lethargy – Plants showing signs of calcium or another mineral deficiencies – Shrimps having difficulty with molting – Shells on snails that are thin, flaking or pitted

Remember that GH measures both calcium and magnesium, so if your water has high GH but you still see these symptoms, it’s possible your water has lots of magnesium but very little calcium. In this case, use a calcium test kit (specifically for fresh water) to determine if you’re lacking that particular mineral.

Bottom line: Don’t let your GH values get too low because it may result in poor growth or even death with your animals and plants.

What is the relationship between pH, KH and GH?

The three ions that are measured in pH, KH and GH are specific types. The release of multiple types ions from a natural mineral source can have a significant impact on water parameters. Limestone, for example, contains a high amount of calcium carbonate. This contains both calcium and carbonate ions, and raises both GH (gravity) and KH (hydrogen). You can only increase GH and not KH if you increase the specific ions of GH (calcium or magnesium), but you should not include ions that have an effect on KH (carbonates or bicarbonates). In fact, African cichlid keepers often buy or create specific salt mixes to individually raise KH or GH.

KH, which is a preventative measure against rapid pH changes, directly correlates with pH. Aquariums have a tendency to lose pH over time. Therefore, if KH is increased, the pH value tends to remain higher. We have observed that when you have a pH higher than 8.0, and add a buffering agent such as crushed coral, KH will increase but the pH value won’t change as much. If you have a lower pH value and add crushed coral, both KH and pH values tend to rise.

How to Change pH, KH, and GH

There are many, many ways to lower and raise the pH, KH, and GH in your aquarium – some that are less effective and others that can be dangerously potent. We prefer to use gentler methods. If you want to lower pH, KH, and GH and soften your water, we recommend letting the tank acidify over time by managing minimal water changes and gradually mixing in water filtered through an RODI (reverse osmosis de-ionized) water system.

Crushed coral can be used to increase pH, KH and GH, or to filter your water. It can be mixed in to the substrate, or used as a bag of media in your hang-on back or canister filters. For fish health, our Washington retail store sells crushed coral. When adding it to your substrate, we recommend starting with 1 pound of crushed coral per 10 gallons of water. To maintain your water’s mineralization, you will need to replace crushed coral every 6-12 months.

Crushed coral

You can also use Wonder Shells and Seachem Equilibrium to harden your water. These supplements may not be required if your water is already hard. However, if your water is already hard, you might be able just to make extra water changes.

Both beginner and veteran fish keepers often take pH, KH, and GH for granted, so don’t fall into the trap of assuming those water parameters are always fine. You’ll be able to catch many problems before they escalate into major disasters if you get into the habit of testing them regularly as preventive maintenance. If you enjoyed this article, don’t forget to sign up for our weekly newsletter so you won’t miss our latest blog posts, videos, and events!